When Depth property is set it is expected that the undo stack would not keep more items than the depth value. However, this behaviour is not working in ImageHistory class and could lead to OutOfMemoryException when the stack gets full of ImageHistoryItem instances.

Steps to reproduce:
1. Open ImageEditor and load some image.
2. Set ImageEditor.History.Depth = 1.
3. Make a few rotations.
Expected: When making Undo you should be able to undo only the last rotation.
Actual: You can Undo all the rotations meaning that Depth property is not respected.